Identification of a conserved locus for jaw laterality. A prominent feature of jaw laterality in Perissodus microlepis is a sided difference in the length of the retroarticular processes (ra) of the jaw (A, B). We found that a similar asymmetry segregates in the F2 from an interspecific cross between two herbivorous species from Lake Malawi (C, D). (E) Linkage analysis revealed a significant association between handedness of retroarticular length and two linked markers on linkage group 10. (F) Larval P. microlepis were typed as either righty (R) or lefty (L) and genotyped at both markers. GM294 was invariant, but UNH2101 showed a size polymorphism that segregated with handedness in this family. Patterns of inheritance are consistent with dominance of the lefty allele and early lethality when the dominant allele is homozygous, which is similar to what has previously been reported by Hori and Hori et al. [14, 19].